(1) Field of the Invention
The invention relates to a method of photoresist stripping, and more particularly, to a method of photoresist stripping which also removes polymer buildup and prevents corrosion in the manufacture of integrated circuits.
(2) Description of the Prior Art
In the manufacture of integrated circuit devices, photoresist is often used as a mask when etching metal to form metal lines. After metal etching, photoresist residue remains on the metal lines. A Cl.sub.2 and BCl.sub.3 plasma is often used in etching the metal lines. Chlorine-containing etchant byproducts form on the sidewalls of the metal lines during etching. If these byproducts are left on the sidewalls of the metal lines, they will react with absorbed moisture from the atmosphere to form hydrochloric acid which will corrode the metal lines. The so-called mouse-bite is a kind of pitting corrosion of the metal sidewalls which is triggered by the post-metal etch treatment and magnified during the subsequent solvent clean process.
It is desired to remove the photoresist residue and the polymer buildup on the sidewalls of the metal lines. In current practice, one solution is to preheat the wafer, then rinse it with water and use oxygen ashing to partially strip the photoresist and to remove the polymer. This prevents the corrosion and mouse-bite problems because it forms a homogeneous and thick native oxide layer at the metal sidewalls, but leaves some of the photoresist residue for a downstream process to remove. Even this partial strip can take a long time, as much as 100 seconds.
Many workers in the art have addressed this problem. Conventional stripping methods use a plasma of oxygen and an oxygen-activating gas such as CF.sub.4. This is sometimes ineffective when a polymeric resist is hardened by exposure to a plasma. Passivation techniques are used to prevent corrosion. The chlorine-etchant byproducts on the sidewalls of the metal lines are exposed to a CF.sub.4 plasma, for example. The CF.sub.4 plasma coats the sidewall polymer, preventing it from reacting to the atmospheric moisture. A wet chemical etchant is often used to strip sidewall deposits. A number of patents use O.sub.2 and CF.sub.4 to remove photoresist and sidewall residues. For example, U.S. Pat. No. 5,545,289 to Chen et al, U.S. Pat. No. 5,382,316 to Hills et al, U.S. Pat. No. 5,174,856 to Hwang et al, U.S. Pat. No. 5,221,424 to Rhoades, and U.S. Pat. No. 5,007,981 to Kawasaki et al all use these two gases. Hwang et al also uses O.sub.2 and NH.sub.3 in a second step. All of these methods use a high power of more than 500 watts. U.S. Pat. No. 5,200,031 to Latchford et al uses NH.sub.3 and O.sub.2 alone to remove photoresist and sidewall residues.